I always find these horn races futile. It is seldom a true measure of actual performance as much as it is ability to anticipate. An SLK55 has no chance of beating my SLK350 in one.
That is not bravado, but is fact. (I plead the fith.) I will explain the how, but the why is simple: an automatic cannot hide it's revs and be in the right gear while a manual can.
(PS: I will not do this on public roads, the track is the only place to prove one's worth.)
If I hear a clever 55 driver in the right gear with the right revs, I am going to smile and wave them off, and they will not get the opportunity to win. Only fighting battles you can win and all that.
If they are loping along, then let them learn the lesson:
1 MPH more speed is 1.5 feet a second. I increase by 1 or 2 MPH before the third horn beep, this is almost undetectable. Clutch in so they do not hear the RPM, then blip and dump right at the third beep (which due to people's subconscious pacing is easy to anticipate) to make the car leap. If you are honking, delay the thrid beep to screw up their preperations.
Now, the 350 has 10 or 15 MPH before the auto trans in the SLK55 hooks up and the 350 has put about 25 feet of distance on the 55 by the time the 55's power comes into play. That distance will take the 55 about 37 more MPH to make up over the 350's current higher speed to catch up, and with the 350 steadily increasing speed, and increasing distance, even if at a slower rate, the SLK55 will have to make great strides.
Before the 55 gets almost even with the 350 (in my expierence up to the 350's rear bumper,) the 350 has reached 100 and can brake and claim victory.
It's a cheap shot, admittedly, but it just goes to prove what Mickey Thompson did a long time ago: a cleaver driver who knows how to bend the rules will be the one who wins. Something I do not usually practice as I prefer honor to glory.
Back to the subject: a properly launched 55 from 30 MPH should have no problem accelerating away from an M3 at speeds under 100 MPH, as the torque numbers would accelerate the weight faster. Horsepower stretches the top end, torque throws you out of the gate.
A dual clutch (not SMG) or a manual M3 from a standing start would be hard for the 55 as the 55 just does not have the axel for it, and has that automatic. An automatic or SMG M3 vs the SLK55, that would be close, I would give the 55 the nod there.